Microdosimetry is a branch of radiological physics that provides a quantitative characterization on micrometric scale of the spatial and temporal distribution of the energy deposition in matter exposed to ionizing radiation. For example, the microdosimetry may provide an indication of the action of the radiation on biological matter (molecules, cells, tissues, and whole organisms, including humans) related to the energy absorbed in subcellular volumes.
Typical applications of the microdosimetry include radiobiology (which seeks to discover the molecular changes responsive to radiation, such as cancer induction, genetic mutations, and cell death), radiation protection (i.e. protection against harmful effects of radiation), and radiotherapy (which is the use of high-energy radiation—from x-rays, gamma rays, neutrons, and other sources—to kill cancer cells and shrink tumors). Moreover, the microdosimetry may also be applied in the study of the effects of radiation on electronic devices.
A microdosimeter typically includes a radiation detector and a measuring system, which is adapted to evaluate a response of the radiation detector to obtain information about the incident radiation. Typically, in the above-mentioned applications to biological matter, the detector is covered by a layer made of a tissue-equivalent material; the tissue-equivalent layer is designed to mimic the response of biological tissue, i.e. to absorb and scatter radiation to the same degree, so as to simulate a radiation field generated under an equivalent biologic tissue when struck by radiation.
As a result, the energy deposited in the detector is related to the so-called linear energy transfer (LET) in tissue. The LET of a charged particle (ion) traversing a microscopic volume is approximated by the quantity lineal energy, i.e. the quotient of the energy deposited in the volume and the mean chord length of the volume. This relation to LET enables microdosimetry to distinguish between recoil electrons, protons, alpha particles and heavy ions. This, in turn, enables the determination of neutron and gamma ray absorbed doses, quality factors, and dose equivalents.
A first experimental detector has been the Tissue-equivalent Proportional Counter (TEPC), which uses a low-pressure gas (with an atomic composition similar to that of biological tissue) to fill a cavity roughly of some centimeters in diameter. The cavity lies at the center of a sphere of conducting plastic (also with an atomic composition similar to that of biological tissue). The TEPC “samples” a particle's track, so that the energy deposited in the gas is related to the LET of the particle. The TEPC presents some disadvantages, such as the impossibility of producing detectors of small sizes that can be easily located in an anthropomorphous puppet or in-vivo on a patient, and the impossibility of observing physical phenomena directly at micrometric sizes without simulating them acting on the gas pressure.
Detectors of the semiconductor type have also been proposed; the semiconductor detectors are stable, linear in their energy response to all types of particles, and can be made small and very thin (less than 0.01 mm). Typically, a semiconductor detector provides a sensitive volume (wherein energy is collected) restricted to the depletion zone around a PN junction. When a particle reaches the depletion zone, it causes the formation of electron-hole pairs and, then, it deposes energy. The depletion zone ensures a minimal recombination of electron-hole pairs and, accordingly, the amount of recombination of charge can be related to the LET of the particle with a high degree of accuracy.
Tests of microdosimetry feasibility have been performed on different semiconductor detectors. The test results agree with the theoretical expectations, but they have also pointed out the influence of the electronic noise, which limits a minimum LET to be detected, and of a so-called field-funneling effect.
The field-funneling effect consists of a transient local distortion of the electric field in the depletion zone, which occurs when a particle's track intercepts a PN junction. The equipotential lines are stretched in the shape of a funnel along the track, and the excess charges produced by the track inside this funneling region are collected very rapidly (typically within a fraction of a nanosecond), which results in a return of the electric field to the steady-state condition. The field-funneling effect is induced by high-LET particles, leading to the collection of electron-hole pairs produced in a non-depleted zone. Accordingly, this effect causes an undesired dependence of the sensitive volume thickness on the particle LET.
For example, let us consider a commercial photodiode Hamamatsu S3590-06, not biased, having a depletion zone of 20 μm and an effective area of about 1 cm2. Test results show that the field-funneling effect has brought the sensitive volume to even double the thickness (40 μm).
It has been also realized a microdosimeter with an ASIC (Application Specific Integrated Circuit) in BiCMOC 0.8 μm technology, including a matrix of PN diodes, each one having a sensitive area of 1 mm2 and a depletion zone of about 2 μm. The electronic noise, measured on the microdosimeter with its measure system, has limited the detectable LET to 10 keV·μm−1. In addition, the field-funneling effect has brought the sensitive volume thickness to about 12 μm.
U.S. Pat. No. 5,854,506 (the entire disclosure of which is incorporated herein by reference) discloses a semiconductor detector of the so-called ΔE-E type. This ΔE-E detector includes a detection cell having a vertical structure consisting of a thick diode (with a thickness of about some hundreds of μm) and a thin diode (with a thickness of about some μm). The two diodes are integrated in a same chip of semiconductor material and have a common anode buried in the chip, a front cathode and a back cathode.
In operation, the two diodes are reverse biased in total depletion conditions. When the detector is irradiated, a particle interacts firstly with the thin diode, losing only a small first part of its energy (ΔE), and then with the thick diode, to which it yields a greater second part of its energy, up to all the residual one (E-ΔE). Accordingly, the detection cell includes two distinct sensitive volumes, a ΔE region and an E region, separated by the common buried anode having a heavily doping concentration; the clear separation between the two regions provides an effective limitation of the field-funneling effect, with the excess charges that are collected into the E region.
A ΔE-E detector for microdosimetric applications typically has a modular structure with a matrix of detection cells; each detection cell has a sensitive volume comparable with the biological cell size, in order to improve the detection efficiency.
However, such a detector shows a high capacitance and, accordingly, a significant electronic noise. Then, the microdosimetric requirement of a detectable LET lower than 10 keV·μm−1 imposes very high performance in terms of noise to circuits coupled to the detector.
Furthermore, the sensitive volume of each detection cell is increased to include a region surrounding each detecting cell; this drawback is particular acute in microdosimetric applications, wherein it is very important to keep the sensitive volume comparable with the biological cell size.